I had this same thought the other night (while drunk/drinking). The stream of consciousness was this:
<First glass of St Bernardus 12 (a 10% abv Belgian quadrupel, for the uninitiated)>
1. Putting out theme list sells models. They encourage using specific models, multiples of specific models, in ways and quantities that they were not being used before. If the theme lists are non-abusive, interesting, and result in shaking up the meta, this is a "good thing".
2. If point 1 is true, especially on the idea that (at least part) of the motivation for theme lists is financial, then putting out more and more theme lists will be the order of the day. Writing rules, especially with the all digital release format, has little development cost but still sells more models.
<Second glass of St Bernardus 12>
3. If points 1 and 2 are true, then Legion certainly has more themes on the horizon!
<Shot of Bulleit to celebrate>
4. Speculation begins... what theme lists are we likely to see? And more importantly, how awesomely over-powered will they be? Much deliberation here.
<Third glass of St Bernardus 12>
5. You only have to look to Cygnar to realize that PP certainly aren't afraid to put out powerful theme lists that do not restrict armies from taking all of the models they would normally anyway. So we began to list the crutch models for Legion: Forsaken, Hellmouth, probably Seraphs. Realizing Hellmouth has yet to be available to any theme list, I posited that the next theme list would be likely include them. Probably some kind of tentacle/mutation/abomination theme list. What other models would such a list have?
<Decadent salted pork-product and burger consumed messily, fourth glass of St Bernardus 12 ordered>
6. Well, the Throne has tentacles, and no one ever buys them because they aren't any good... wait a second, aren't they under CID? Didn't I post in that thread? Do I drink too much?
<Too full for more beer, I moved onto the hard-liquor phase of the evening with a Jack & Coke>
7. Checking out that thread on my phone, I discover I actually started that thread. Ok, so they are going to put out new rules for the Battle Engine... and all the other Battle Engines to boot... wouldn't be at all surprised if they then put out some kind of theme lists for everyone pushing the new Battle Engine designs.
<Mid-way through Oujia: The Origin of Evil, terrified by little girls crawling on the ceiling, my thoughts turned to other scary things, while enjoying my second and then third Jack & Coke>
8. Oh firetruck me! The Battle Engine and the Hellmouth look really freaking similar. What if the Hellmouth is like the immature form of a Battle Engine? It eats and gestates underground, and then bursts forth in majestic glory. For Vayl to ride on. Why isn't Vayl riding on a Throne? Wait, Vayl 3...
9. Why am I in bed? Who put my pants back on? Is there any Jack left?